Draft tube for catalyst rejuvenation and distribution

ABSTRACT

Catalyst in a slurry phase reactor is rejuvenated and uniformly distributed in said reactor using a substantially vertical draft tube fully immersed in the slurry which utilizes a rejuvenating gas injected substantially near the bottom of the substantially vertical draft tube whereby catalyst near the bottom of the slurry phase reactor is drawn up the draft tube and discharged from the top of the draft tube near the top of the slurry phase in said reactor.

FIELD OF THE INVENTION

This invention relates to a process and apparatus for regenerating -rejuvenating and uniformly distributing catalyst in a slurry phasereactor by using a substantially vertical draft tube means, open at bothends, fully immersed in the slurry in said reactor and utilizingrejuvenating gas injected at or substantially near the bottom of saiddraft tube means through hydrogen gas injection means. Catalyst is drawnup the draft tube means from near the bottom of said reactor under theinfluence of the rejuvenating gas and ejected from the top of the drafttube means at or below the top of the slurry phase in such reactor.Catalyst reactivation - regeneration is accomplished using the drafttube means by using a rejuvenating gas such as hydrogen. For the purposeof this specification, draft tube means will be referred to variously asdraft tube, draft tubes, rejuvenation tube or rejuvenation tubesaccording to the context of the specification, unless otherwiseindicated.

BACKGROUND OF THE INVENTION

Slurry reactors are well known for carrying out highly exothermic, threephase, catalytic reactions. Usually called “bubble columns” thesereactors have a liquid phase in which solid catalyst particles aredispersed or held in suspension by a gas phase bubbling through theliquid phase, thereby creating a slurry. These reactors provide improvedheat transfer characteristics for the exothermic reaction, with thebubbling gas maintaining the catalyst as a dispersion in the liquidphase.

Bubble column reactors typically have a multiplicity of tubes suspendedwithin a shell-type housing, the tubes being filled with a heat transfermedium, e.g., boiling water, which absorbs the heat generated by theexothermic reaction occurring on the shell side of the tubes in the mainbody of the housing.

Alternatively the reactor can be of a similar multitube design housed ina common shell-type housing as previously described but wherein the gasand liquid are passed through the multiple tubes which function as thereactor tubes, with effluent being removed from the upper ends of thereactor tubes and heat transfer fluid being passed through the spacealong the outside surfaces of the reactor tubes. The reactor tubes canbe either multiple individual tubes with spaces between adjacent tubes,or multiple bundles of tubes with spaces between adjacent bundles oftubes.

Likewise the entire cross section of the reactor vessel may have aplurality of shafts disposed within it, the bottoms of said shafts beinglocated above the reaction gas inlet but extending a distance above thetop surface of the reaction slurry into the gas disengaging spaces so asto create multiple single columns of standing, noncirculating liquidwith catalyst suspended and dispersed in said standing liquid. Thereaction zone therefor has multiple single columns, said columns havinga common bottom reaction gas introduction zone and a common upper gasdisengagement space. To insure proper control of the exothermic processadditional tubes can be inserted into or between the multiple singlecolumns to function as heat exchangers.

It would be an advance if, in whatever configuration the reaction vesselmay take, catalyst within the reaction vessel could be more uniformlydistributed and circulated so as to insure more even catalyst aging inthe course of the reaction, more effective use of the catalyst byinsuring a higher probability that the maximum amount of availablecatalyst is in the reaction zone to promote the reaction by eliminatingstagnant zones of uncirculating, standing catalyst, decreasing masstransfer limitations, and improving heat transfer utilization.

DESCRIPTION OF THE FIGURES

FIGS. 1, 2 and 3 present in graphical form the results of cold mock-updraft tube runs comparing slurry distribution in vessels with andwithout the use of added lift gas.

FIG. 4 presents axial catalyst distribution comparisons at specificmeasurement moments before and during draft tube operation in anoperating bubble column reactor (4 draft tubes and one rejuvenation tubein use).

FIG. 5A presents catalyst activity and FIG. 5B shows hydrogen flow ratesin continuous catalyst rejuvenation. FIG. 5A presents the efficacy ofusing draft tubes as continuous catalyst rejuvenation tubes usinghydrogen as rejuvenating lift gas.

FIGS. 6A, B and C present three pairs of temperature profiles comparingtemperatures inside the rejuvenator tube with temperature in the reactorslurry outside the rejuvenator tube at different temperatures in thereactor slurry.

SUMMARY OF THE INVENTION

Catalysts used in slurry phase reactors, such as hydrocarbon synthesiscatalyst used to produce hydrocarbons from synthesis gases, or methanol,which have become reversibly deactivated during use are regenerated -rejuvenated, circulated and uniformly distributed throughout the slurryphase reactor by use of a substantially vertical draft tube, open atboth ends, fully immersed in the reaction slurry, the bottom of whichdraft tube preferably extends to near the bottom of the slurry reactorand the top of which preferably extends to just under the top of theslurry phase, utilizing a rejuvenating lifting gas injected into thedraft tube at or substantially near the bottom of said draft tube.

The draft tube is sized in terms of length and diameter so as to insurethat flow in said tube is at or above that flow which provides bothcatalyst lift and catalyst rejuvenation. Velocity of the rejuvenationgas in the draft tube is such that the slurry density in the draft tubeis less than the slurry density in the overall reaction vessel.Superficial gas velocities in the tube, therefore, are at least 0.2 to40 times the superficial gas velocities of the gases in the reactorvessel itself, preferably 0.5 to 20 times, more preferably 3 to 15 timesthe superficial gas velocities of the gases rising in the reactorvessel.

The draft tubes of the present invention are sized so as to fit withinthe reaction vessel and are also sized so as to not interfere with thefluid dynamics of the vessel nor with the normal synthesis gas flowwithin such vessel. These draft tubes occupy, on a cross sectional areabasis, as measured in the horizontal plane through the vertical drafttubes, a total of from 0.2 to 10% of the cross sectional area of thereaction vessel, preferably from 0.4 to 8%, more preferably from 0.4 to5% of the cross sectional area basis of the reaction vessel. Ideallymultiple tubes will be employed as to insure maximized catalystcirculation. When multiple tubes are employed no single tube willconstitute more than 50%, preferable more than 30%, more preferably morethan 10% of the total cross sectional area of the draft tube array.

Narrower diameter tubes are preferred so that fluid dynamics are moreeasily controlled and so that excessively high superficial gasvelocities to achieve adequate lift can be avoided. Within the crosssectional area constraints recited above, tubes having diameters of lessthan 12 inches, preferably less than 8 inches, more preferably less than6 inches will be employed in commercial hydrocarbon synthesis vessels.

The length of the tube is important, since when all other conditions,are constant, it is believed the amount of slurry pumped by the drafttube increases as length is increased. Thus the length of the lift tubewill be as long as the reactor design allows, i.e., approximately equalto the slurry height in the reactor. The diameter will be set by flowregime considerations in the lift tube and by the amount of slurry thatis to be pumped. Successful draft tube operation depends upon thedensity of the gas-liquid-solid slurry inside the draft tube being lessthan the density of the gas-liquid-solid slurry in the reactorsurrounding the draft tube. The greater the difference is in these twodensities, the higher is the velocity in the draft tube.

The density inside the draft tube depends upon the flow regime therein,and that in turn depends upon the draft tube diameter and gas velocity.Furthermore, there is probably some interaction between diameter andvelocity. That is to say, an acceptable gas velocity range in a smalldiameter tube may be different from that in a larger tube, because thedifferences in densities between the draft tube and reactor slurrieswill be different for different draft tube diameters, at a givendifference in velocity between the draft tube and reactor.

To be effective in catalyst dispersion and rejuvenation, the upwardvelocity of the fluid in the draft tube must be greater than thesettling velocity of the solids, otherwise the solids will not becarried up the draft tube. At the other extreme, too high a gas velocitywill cause the flow regime to become annular in which the liquid-solidphase is spread out as an annulus against the wall of the draft tubewith the gas passing at high velocity inside the liquid-solid annulus.Between these two extremes of gas velocity, the draft tube goes throughan optimum operating region for catalyst dispersion. As the gas rate isincreased from a low level, the rate of slurry (liquid+solids) pumpingfirst increases, thereby improving the solids dispersion. As the gasrate is increased further, the pumping rate goes through a maximum andbegins to decrease as the gas rate is increased further. This wasobserved in the mockup of Example 1 (see FIG. 3), discussed in greaterdetail later, when the gas rate was increased from 0.4 to 0.8 CFM(superficial gas velocity in the tube increased from 46 to 92 cm/sec),the catalyst dispersion was poorer at the higher velocity.

Hydrogen, or such other hydrogen rich gas which may contain inerts suchas CH₄, light hydrocarbons (e.g., C₂-C₁₀) etc., but which issubstantially free of CO or other hydrocarbon synthesis process feedgases which are reactive with hydrogen, is used in the draft tube ascatalyst rejuvenation gas and lifting gas. It has been discovered thathydrocarbon synthesis catalyst which has undergone short term reversibledeactivation in the course of the HCS process can be reactivated in thepresence of the hydrocarbon synthesis product using hydrogen, saidcatalyst rejuvenation occurring under the conditions of temperature andpressure similar to those employed for the hydrocarbon synthesis.Catalyst regeneration - rejuvenation using hydrogen or hydrogencontaining gas is the subject matter of copending application Ser. No.949,934 Filed Sep. 24, 1992 in the name of W. N. Mitchell.

To permit the draft tubes to function as catalyst rejuvenation zones thedraft tube is fitted at its lower end with gas deflecting means such asa baffle which curtails entry into the tube of synthesis gases yetpromotes or facilitates entry of additional liquid and catalyst(slurry). With such synthesis gas influx interdicted, the catalyst andsynthesis product liquid present in the tube can be exposed to thehydrogen gas stream injected into the draft tube at or substantiallynear the bottom of the tube. Because the tube is fully immersed in thereaction slurry, the temperatures and pressures exerted on the contentsof the draft tube are those of the synthesis process.

The amount of hydrogen flow into the tube when used as a rejuvenationtube can be throttled such that at the beginning of the regeneration -rejuvenation step flow is low enough so that minimal catalyst isdisplaced out of the tube through the open top. Flow is maintained atthis level for a time sufficient to effect catalyst rejuvenation afterwhich hydrogen flow is increased to lift the catalyst out of the tube topermit a fresh charge of additional catalyst and hydrocarbon synthesisproduct to be drawn into the tube. Alternatively, hydrogen flow rate isadjusted so that catalyst is continuously being drawn into the tube fromthe bottom in response to the hydrogen lifting flow; catalyst residencetime in the tube is sufficient to achieve the regeneration -rejuvenation of the catalyst by the time any particular catalystparticle has completed its journey to the top of the tube for dischargeback into the main reactive slurry.

The extent of the rejuvenation reaction occurring in the continuous modeusing the draft tube as the rejuvenation vessel can be monitored bythermocouples placed inside the tube. The measured temperature profilein the rejuvenation tube is compared with the temperature profile in thereactor slurry surrounding the rejuvenation tube, correspondingthermocouples inside and outside the tube being at equivalent heightsabove the bottom of the reaction vessel. The difference in temperaturebetween the contents of the rejuvenation tube and the reactor slurry isthe temperature rise in the rejuvenation tube, which can be used as ameasure of the extent of the rejuvenation reaction occurring there. Theefficacy of continuous rejuvenation in the rejuvenation tubes dependsupon the temperature level in the rejuvenation tube which is controlledto some extent by the temperature in the reactor slurry itself. Aspreviously stated, depending on the level of deactivating species on thecatalyst or in the wax, rejuvenation at higher temperatures ispreferred.

When catalyst activity is low, indicating that the concentration of thedeactivating species in the wax and on the catalyst is high, the amountof reaction that occurs in the rejuvenation tube must also be high, andis evidenced by a greater temperature rise in the rejuvenation tube.When there is only little deactivation, the temperature rise in therejuvenation tube is proportionally smaller. Hydrogen gas rate to therejuvenation tube determines the residence time of the reactor slurry inthe rejuvenation tubes and is important in determining the efficacy ofthe rejuvenation. Controlling residence time of the fluids in therejuvenation tubes is effected by controlling the amount of hydrogen gasbeing fed to the tube. Too high a rate of hydrogen reduces the residencetime in the tube to a point that insufficient time is available for thepre-clean up and clean-up reactions to occur.

The amount of hydrogen passed to the tube in the rejuvenation mode so asto effect sufficient residence time depends on the degree or level ofcatalyst deactivation, the concentration of deactivating species in thewax present in the slurry, the diameter of the tube, and are all itemseither within the control of the practitioner or dictated by theconditions of the synthesis reaction itself. Thus, control of hydrogenflow rates to the rejuvenation tube is left to the individualpractitioner to set in response to the specific conditions encountered.When used for rejuvenation, the rejuvenation tube can occupy from 0.2 to10% of the cross sectional area of the reaction vessel.

As disclosed and claimed in copending application Ser. No. 994,219 filedeven date herewith in the names of Behrmann and LeViness, degree ofcatalyst rejuvenation in the rejuvenation tubes can be controlled byindependently controlling the rejuvenation temperatures in therejuvenation tube as compared to the temperature of the surroundingreaction slurry. In many instances this involves conducting therejuvenation at temperatures higher than those of the surroundingreactor. This control of the temperature in the rejuvenation tubes canbe achieved either by increasing the residence time in the rejuvenationtube, so as to take advantage of the exothermic nature of therejuvenation process itself and thereby increase the temperature, bydeliberately introducing heat into the rejuvenation tube, by acombination thereof, or by introducing a cooling medium into therejuvenation tube, thereby lowering the rejuvenation temperature.

The temperature in the rejuvenation tube should be high enough to reactout any entrained and dissolved CO in the lower part of the rejuvenationtube and react deactivating species in the wax and on the catalyst, yetlow enough to avoid excessive methane production and hydrolysis of thewax. The rejuvenation temperature in the rejuvenation tubes to achieveeffective catalyst rejuvenation may range from about 400° to 500° F.,preferably about 420° to 480° F. and more preferably about 440°-470° F.The lower temperatures are effective in those instances in which thecatalyst and/or wax contain a minimum of deactivating species. Highertemperatures are needed in those instances when the catalyst and/or waxcontaining higher levels of deactivating species.

To effectively take advantage of the heat produced by the exothermicnature of the rejuvenation process itself in the rejuvenation tubes, itis preferred that the rejuvenation tube be fitted with insulation means,thus trapping the heat in the rejuvenation tube. This insulation meanscan take the form of a coating of material of low heat transfercoefficient, such as ceramic. Alternatively the rejuvenation tube can besurrounded by a larger diameter tube with the annular space between therejuvenation tube and the larger diameter tube surrounding it thusisolating it from the reaction slurry.

Alternately, heat or cooling can be introduced into the rejuvenationtube by means of a separate, independent, controllable heating orcooling means source, such as a steam heat exchanger or electricalheater, run partially or totally up the interior of the rejuvenationtube. When heating, it would be preferable to provide the maximum heatexchange near the bottom of the rejuvenation tube to provide the maximumbenefit in increasing the rate and extent of rejuvenation.

When using the independent heat source/heat exchange inside therejuvenation tube, it is preferable to simultaneously employ aninsulating wrap around the rejuvenation tube.

In this and the previous embodiment the heat exchanger extending totallyup the inside the rejuvenation tube might serve the purpose of heatingthe contents of the rejuvenation tube in the lower region and mitigatingthe temperature rise (i.e. cooling) in the upper region, should reactionrates and heat of reaction be high enough to cause the temperature inthe upper regions to rise to undesirable levels.

Ideally catalyst distribution and rejuvenation will be practicedsimultaneously using the same draft tubes. When multiple draft tubes areused for catalyst redistribution some of the tubes may be fedrejuvenating gas at a high enough superficial velocity for the purposeof accomplishing both catalyst rejuvenation and redistribution.

When a number of draft tubes are employed as an array, those which areused solely to accomplish catalyst redistribution can be fed lift gasother than just hydrogen or hydrogen containing gas. Non-rejuvenatinglift gas can be any gas such as gas feed, tail gases, volatile liquidproduct, light gaseous hydrocarbons, inert gases such as nitrogen etc.,steam. When used for catalyst redistribution the superficial gasvelocity in the tube can be in the range of at least 0 to 40 times,preferably 2 to 20 times, more preferably 3 to 15 times the superficialgas velocities of the reaction gases rising in the reactor vesselitself.

As previously stated, the draft tubes are also located in the reactionprocess zone so as to produce uniform catalyst redistribution throughthe reaction zone and mitigate or eliminate areas of catalyst stagnationand overcome the natural settling tendency of the catalyst that createsa higher concentration of the catalyst in the bottom of the reactor thanat the top. Thus the lower ends of the draft tubes will be placed at ornear the bottom of such reaction zones in those areas of low or minimalnormal circulation in said zone, preferably from 0.1 to 1.0 foot fromthe bottom of the reaction zone, more preferably from 0.1 to 0.5 footfrom the bottom, most preferably from 0.1 to 0.25 foot from the bottomof the reaction zone. Such stagnant zones exist in bubble columnreactors wherein the catalyst is on the shell side in a shell and tubereactor. Bubble column synthesis gas is introduced into such reactor bygas introduction means such as bubble caps at the bottom of the reactor.Due to fluid dynamics stagnant zones are present at the bottom of thereactor surrounding the gas introduction. Catalyst accumulating in thosezones is not circulated or lifted by the incoming synthesis gas; suchcatalyst in effect is lost to the catalytic process. With more advancedgas introduction/distribution means such as multiple cone distributors,stagnant zones of uncirculating, standing catalyst are avoided, but poorcatalyst distribution throughout the slurry remains a problem.

The catalyst maldistribution problem revolves around the axial gradientof catalyst concentration. While the energy impacted by the gas bubbletends to disperse the catalyst, gravity causes the catalyst to settle.The degree of dispersion increases with increasing gas velocity,increasing liquid velocity in the upward direction, increasing liquidviscosity, increasing liquid density, and decreasing particle size. Forpractical conditions encountered in commercial vessels, there is still alarge gradient of catalyst concentration from the bottom to the top ofthe reactor even when multiple cone distributors are used so that thereare no stagnant standing zones. It is this gradient which is flattenedusing the draft/rejuvenation tubes.

In the case of draft/rejuvenation tubes, catalyst is carried by therejuvenating gas from the high concentration zone in the bottom of thereactor to the low concentration zone at the top of the reactor. Gravityslowly pulls the catalyst particles back to the bottom of the reactorwhere they are again picked up and lifted to the top.

Siting draft tubes around the e.g., bubble caps in such reactors wouldresult in a siphoning of the catalyst up from the static zone into thedraft tube in response to the suction created in the draft tube and thedischarge of such formerly static catalyst out the top of the draft tubeback into the main reactive slurry mass.

In reactors which are not of the bubble column design but are still of aslurry design employing the gas introduction means described abovewherein reaction still occurs on the shell side of any columns in thereactor, similar stagnant zones or concentration gradients exist eventhough such designs may have associated with them a high degree of backmixing. Eddies can be and are created which create relatively stagnantcatalyst zones. Such zones and gradients can also be effectivelyaddressed using the draft tube/lifting gas assembly of the presentinvention.

As stated, the present invention is of use in hydrocarbon synthesisprocesses wherein gas, i.e. hydrogen and carbon monoxide, in a ratioranging from about 0.5 to 4, preferably 0.7 to 2.75, more preferablyabout 0.7 to 2.5, or other synthesis feed such as methanol, is injectedat superficial gas velocities ranging from about 1 to 20 cm/sec throughgas injection means such as a bubble cap gas injector grid, or spargerinto the main reaction zone in which is located hydrocarbon synthesisproduct (i.e. hydrocarbon liquids or liquid wax) and catalyst. The gasbubbles up through the reaction zone in contact with the catalyst in thehydrocarbon liquid and is converted into hydrocarbon product. The risingsynthesis gas supplies the energy to maintain the catalyst as adispersion in the hydrocarbon liquid thereby creating a slurry.

Reaction takes place wherever there are synthesis gas, catalyst andsuitable reaction conditions, which include pressures ranging from 1 to100 atmospheres, preferably 10 to 50 atmospheres, more preferably about15 to 40 atmospheres and temperatures ranging from about 175° C. toabout 450° C., preferably about 175° C. to 420° C., more preferablyabout 175° C. to 300° C.

The slurry phase liquids in which the catalyst is dispersed are thosethat are liquid at reaction conditions, generally inert, and a goodsolvent for synthesis gas. Typically, the slurry is the product of thereaction and contains C₅₊ hydrocarbons, usually C₅-C₁₀₀ hydrocarbons.Preferably, however, the slurry liquid comprises primarily high boilingparaffins with small amounts of primary and secondary alcohols, acids,esters, or mixtures thereof. Sulfur, nitrogen, phosphorus, arsenic, orantimony heteroatoms are to be avoided since these tend to poison thehydrocarbon synthesis catalyst. Examples of specific slurry liquids aredodecane, tetradecane, hexadecane, octadecane, tetracosane, and thelike. Preferred slurry materials are Fischer-Tropsch waxes and C₁₆-C₁₈hydrocarbons.

The concentration of solids, including catalyst, in the slurry phase isusually about 10-15% by weight, preferably 20-40 wt % solids.

The hydrocarbon synthesis reaction is highly exothermic and the heat ofreaction is removed by a heat transfer material which is eithercirculating on the shell side of a shell and tube reactor when thereaction takes place in the tube, or through the tubes when the reactiontakes place on the shell side. The heat transfer material can be anymaterial having a high heat capacity, whether or not it undergoes aphase change. Preferably the heat transfer fluid is boiling water.

The catalyst employed in the hydrocarbon synthesis process is anycatalyst known to be active in Fischer-Tropsch synthesis. For example,Group VIII metals, whether supported or unsupported, are knownFischer-Tropsch catalysts. Of these, iron, cobalt and ruthenium arepreferred, particularly iron and cobalt, most particularly cobalt.

A preferred catalyst is supported on an inorganic refractory oxideselected from Groups III, IV, V, VI, and VIII of the Periodic chart ofthe elements. Preferred supports include silica, alumina,silica-alumina, the Group IVB oxides, most preferably titania (primarilyin the rutile form), and generally supports having a surface area ofless than about 100 m²/gm, preferably 70 m²/gm and less.

The catalytic metal is present in catalytically active amounts, usuallyabout 100 wt %, (the higher concentrations being typical when iron basedcatalysts are employed), preferably 2-40 wt %, more preferably about2-25 wt %. Promoters may be added to the catalyst and are well known inthe Fischer-Tropsch catalyst art. Promoters can include ruthenium (whenit is not the primary catalytic metal), rhenium, hafnium, cerium, andzirconium, and are usually present in amounts less than the primarycatalytic metal (except for ruthenium which may be present in coequalamounts), but the promoter:metal ratio should be at least about 1:10.Preferred promoters are rhenium and hafnium. Useful catalysts aredescribed in U.S. Pat. Nos. 4,568,663; 4,663,305; 4,542,122.

Catalyst particle size is important and particle sizes may range fromthat which is reasonably separable from the synthesis product to thatwhich is reasonably able to be dispersed in a slurry phase. Particlesizes of 1-200 microns, preferably about 20 to 150 microns meet theserequirements. Particles of this size which are easily separable from thesynthesis product are those most advantageously benefitted by use ofdraft/rejuvenation tubes to provide improved dispersion. Particles ofthis size tend to be more influenced by gravity than are smallerparticles which tend to stay in suspension and not settle out.

Catalyst preparation may be accomplished by a variety of techniques,although catalyst preparation does not play a part in this invention andthe regeneration - rejuvenation treatment disclosed herein will improvethe activity of the hydrocarbon synthesis catalyst however it isprepared.

A typical catalyst preparation may involve impregnation, by incipientwetness or other known techniques of, e.g., a cobalt nitrate salt onto atitania, silica, or alumina support, optionally followed or proceeded byimpregnation with a promoter material, e.g., perrhenic acid. Excessliquid is removed and the catalyst precursor dried at 100° C. to 125° C.Following drying or as a continuation thereof, the catalyst is calcinedat about 300° C.-500° C. to convert the salt or compound to itscorresponding oxide(s). The oxide is then reduced by treatment withhydrogen or a hydrogen containing gas at about 300° C.-500° C. for aperiod of time sufficient to substantially reduce the oxide to theelemental or catalytic form of the metal. Some prefer an additionalcycle of oxidation/reduction. Another, and sometimes preferred methodfor catalyst preparation is disclosed in U.S. Pat. No. 4,621,072incorporated herein by reference.

Examples Example 1

A number of ambient temperature mock-up draft tube demonstrations wereperformed to demonstrate the ability of draft tubes to redistributecatalyst in a reaction vessel environment. Various runs were conductedin a demonstration apparatus comprising a main vessel having an internaldiameter of 5.75 inches in which was located a draft tube of 0.9 inchinternal diameter, the draft tube occupying, in cross sectional areaabout 2.4% of the total cross sectional area of the reactor.

The draft tube was about 12 feet tall and extended from about 0.5 inchabove the bottom of the main vessel and ended below the level of thehydrocarbon slurry, which level differed from run series to run series.

The liquid phase of the slurry consisted of predominantly C₁₃H₂₈ linearparaffin, which has viscosity, density, and gas hold-up propertiessimilar to the liquid product present under hydrocarbon synthesis (HCS)conditions. Catalyst (12% Co - 1% Re on 94% TiO₂-6% Al₂O₃, 50% porosity,4.2 g/cc skeletal density) was used as the solid phase in the slurry.

FIGS. 1, 2 and 3 report the results of these demonstrations.

In each figure a series of runs were conducted.

In FIG. 1, there was an average solids concentration of 26 weightpercent in the slurry and 33% gas hold up.

In FIG. 2 gas hold up was about 25%, superficial gas velocity was 11.3cm/sec with a total slurry height of 177 inches.

In FIG. 3 gas hold up was about 25%, superficial gas velocity was 5.6cm/sec with a total slurry height of 162 inches.

Base line runs were conducted at different gas flow rates (no lift gas)to establish the normal slurry distributions. Additional runs wereconducted in which the draft tube was employed using a lifting gas toshow the effect on slurry distribution.

In all instances for the particular draft tube used in this example, theruns in which a draft tube was employed using a lifting gas having asuperficial velocity greater than at least 1.5 times the superficialvelocity of the main feed gas stream, showed an improvement in slurrydistribution. When lift gas superficial velocity was in excess of 15times than the superficial velocity of the gas feed stream, dispersiondecreased indicating that dispersion goes through a maximum. The mosteffective dispersion is represented by the line closest to horizontal,representing almost uniform catalyst distribution across the vesselheight. The bottom of the draft tube was in a “J” bottom feedconfiguration. This configuration operates as a type of baffle toprevent gas from entering the lift tube through its bottom slurry inlet.Physically the “J” bottom feed configuration it achieved by welding twopipes together at a 90° angle. The top half of the horizontal pipesection is removed to allow gas-free solid-liquid slurry to enter thedraft tube.

In the following Examples 2 and 3 reference is made to differentbalances made at different times during the operation of a hydrocarbonsynthesis (HCS) pilot plant. The runs used a catalyst comprising 12%Co-1% Re on a support of 94% TiO₂-6% Al₂O₃, which was activated byreduction in hydrogen at about 350° C. The liquid phase of the slurryconsisted of the HCS wax product which is liquid under the reactionconditions of 210°-230° C., 20 atm. pressure. Feed gas composition wasabout 56% H₂-26% CO-13% CO₂ - 5% CH₄ (by volume). Tail gas was used asfeed to the draft tubes when employed. Pure hydrogen was fed to therejuvenation tubes. An array of cooling water tubes was present in thereactor to remove the heat of reaction. Table I presents the differentbalances and the conditions employed during each balance, the number ofdraft tubes and/or regeneration tubes in use, the gas velocities in thetubes, the solids concentration reactor densities and reactor axialtemperatures within the reactor slurry at different elevations withinthe reactor vessel for each balance. Reactor Productivity refers to thevolume of CO converted per hour per volume of slurry (catalyst+wax+gas).

TABLE 1 CONDITIONS FOR DRAFT TUBE EXAMPLES TABULATED RESULTS HCS-PDURun-Balance 11 47 58 70 41 Draft Tubes in Service 0 0 0 1-3″φ, 1-4″φ2-3″φ, 2-4″φ Rejuvenation Tubes in Service 0 1-3″φ 1-3″φ 2-3″φ 1-3″φVelocities, cm/sec Reactor Inlet 12.3 14.3 14.6 14.3 13.7 Outlet 10.611.8 12.1 11.5 10.9 Draft Tube 7.8 8.0 7.3 58.9 60.1 Rejuvenation Tubes0 75.9 74.8 36.9 75.7 Reactor Productivity, Vol CO/Hr/ 41 61 61 70 69Vol Slurry Solids Concentrations, Lb Catalyst/(Lb Catalyst + Lb Wax)Elevation. Ft.  0.23 0.4276 0.4030 0.4518 0.4140 0.3361  2.52 0.42020.2820 0.3627 0.2960 0.2275  5.47 0.3340 0.2843 0.3189 0.2462 0.2273 9.41 0.2329 0.2185 0.2380 0.2158 0.2053 13.49 0.1835 0.1965 0.19940.2202 0.2095 20.49 0.0969 0.1690 0.1127 0.1624 0.2000 30.47 0.06600.1178 0.0969 0.1497 0.1860 Reactor Densities, Lb/Cu. Ft. Elevation, Ft. 0.0-2.5 49.27 36.39 37.65 30.29 31.13  2.5-9.8 35.53 28.54 28.74 26.5626.10  9.8-19.8 25.03 23.87 20.82 22.21 22.83 19.8-29.8 21.29 21.6217.91 20.63 21.15 29.8-35.3 9.30 19.16 16.87 21.45 20.35 35.3-39.8 04.94 1.12 1.0 1.92 39.8-48.8 0 0 0 0 0 Reactor Axial TemperatureProfile, ° F. Elevation, Ft.  1.0 415 415 424 424 413  2.0 418 416 425425 414  3.0 421 417 427 427 416  4.0 422 417 428 427 416  5.0 424 418429 429 418  6.0 423 417 427 428 417  7.0 424 417 427 428 417  8.0 425417 427 428 417  9.0 425 417 426 428 417 10.0 425 416 425 428 417 11.0426 417 426 429 419 13.0 424 417 423 427 417 15.0 424 416 423 428 41817.0 423 414 421 427 417 19.0 423 414 421 428 418 21.0 423 414 421 428418 23.0 422 413 419 427 417 25.0 422 413 419 428 417 27.0 422 413 418427 418 29.0 421 412 417 426 417 31.0 421 413 417 427 418 33.0 413 417428 418 35.0 410 414 425 414 37.0 39.0

Example 2

The efficacy of using draft tubes for enhanced catalyst circulation wasdemonstrated in a hydrocarbon synthesis pilot demonstration unit whichis 4 feet in diameter and had a reaction slurry height of about 35 feet.

Balance 11 was made at the start of run, no lift gas was injected intoany draft tubes and no rejuvenation tubes were in use.

Between days 6 and 7 in the run (Balance 41), about 25,000 standardcubic feed per hour of HCS product gas was recycled to four draft tubes,two of which were 3″ diameter pipe and two of which were 4″ diameterpipe, giving a superficial gas velocity of about 2 feet/sec in the drafttubes. In total the 4 draft tubes, having a total cross sectional areaof 39.28 sq. inches occupied only 2.17% of the total cross sectionalarea of the reaction vessel (1809.21 sq. inches). A pair of 3″ diametertubes (14.12 sq. inches) occupied only 0.78% while a pair of 4″ diametertubes (25.12 sq. inches) occupied only 1.39% of the total crosssectional area of the vessel. A pair of tubes made up of only one 3″ andone 4″ diameter tubes had a total cross sectional area of 19.62 sq.inches and occupied only 1.08% of the total cross sectional area of thevessel.

Referring to Table I “Reactor Densities”, a higher density readingindicates a higher catalyst loading. These density readings show thatduring the measurement period Balance 41 between days 6 and 7 duringwhich the lift gas rate was about 25 KSCF/hr, density readings in thebottom of the reactor fell dramatically and the density reading near thetop of the slurry increased as compared to catalyst distribution anddensity reading reported for Balance 11, no lift gas in use.Furthermore, the four lower densities were very similar while the drafttubes were in service. These density changes were the result of thecatalyst being much more uniformly distributed throughout the reactor.

This change in catalyst loading is shown graphically in FIG. 4. ThisFigure shows the catalyst concentration, expressed as lb catalyst per lbof slurry (catalyst plus wax), plotted against elevation in the reactor.Balance 11, a measurement made at 1.62 days, before the draft tubeexperiment was carried out, shows a typical catalyst distributionwithout the draft tubes in which there was almost a 10-fold change incatalyst concentration across the length of the reactor. However, inBalance 41, a measurement made at day 6.41 in the middle of the drafttube experiment, with all 4 draft tubes in use (plus one additional tubeof 3″ diameter used for in-situ catalyst regeneration (see Example 3))and at a lift gas superficial velocity of 60.1 cm/sec, the catalystconcentration was nearly uniform from the 2.5- to the 30.5- foot level.The catalyst concentration at the 0.2-foot level was not as dramaticallyaffected because the lower end of the lowest lift tube was positioned atthe 0.5-foot level and therefore was not completely effective inlowering the catalyst concentration at the 0.2-foot level. In the regionof the reactor over which the draft tube was operating, the catalystconcentration was nearly uniform, thus proving the effectiveness of thedraft tube concept.

The draft tube concept was again demonstrated between days 10 and 12 inbalance 70. This time only two lift tubes (one 3″ and one 4″ diameter)and two regenerator tubes of 3″ diameter were used with a lift gassuperficial velocity of 58.9 cm/sec in each draft tube thus reducing thetotal lift gas employed by a factor of two. Rejuvenation gas rate was36.9 cm/sec. Comparison of Balances 70, 47, 58 and 41 shows that thereactor densities were improved nearly to the same extent as when fourdraft tubes were used. The use of two draft tubes (and two regeneratortubes) Balance 70 definitely improved the catalyst dispersion over thatobtained without the draft tubes Balances 58 or 47, but the benefit wasnot as great as that achieved by four draft tubes plus one rejuvenatortube Balance 41, the total being of greater overall diameter and whichoccupied a higher percentage of the cross sectional area of the reactorvessel. With four draft tubes, the ratio of the concentration at thebottom of the reactor (2.5-foot level) to that at the top of the reactor(30.5-foot level) was less than 1.3, while with the two draft tubes(plus 2 regenerator tubes) the ratio was 2. The concentration at thevery bottom of the reactor (0.2-foot level) was also significantlyhigher with the two tubes than with the four tubes (42 wt % vs. 33 wt%).

The use of the draft tubes to improve the catalyst dispersion alsoflattened the axial temperature profile in the reactor. This is shown inthe case of four lift tubes and one 3″ diameter regeneration tube(balance 41) vs Balances 47 or 58 and for the case of two lift tubes(balance 70 as previously discussed) as compared with balance 58 inwhich no lift tubes were used (but using regeneration tubes of 3″diameter). Balance 58 data show that without the use of the draft tubesthe temperature difference between the top and bottom of the reactor isover 12° F., while the temperature difference when four draft tubes wereoperated (Balance 41) was actually negative because of the lowertemperature in the bottom of the reactor caused by the cooling effect ofthe incoming gas feed. For Balance 70 with two draft tubes and tworejuvenator tubes in operation there was perhaps a 2° F. differencebetween the top and bottom of the reactor.

These two examples demonstrate that very modest sized draft tubesoccupying less than three percent of the reactor cross section veryeffectively improve the dispersion of the catalyst. The benefits ofimproved catalyst dispersion are: (1) reduced mass transfer limitationsand thereby improved catalyst utilization, and (2) improved temperaturedistribution that reduces the selectivity to unwanted lighter productsand that improves the utilization of the heat transfer area in thereactor.

Example 3

Lift tubes were employed to demonstrate the operability of continuouscatalyst rejuvenation during the same set of runs used to demonstratethe efficacy of such tubes for catalyst redistribution using the sameapparatus described in Example 2. The rejuvenation tubes, however, areseparate, distinct, and independent of the four previously describeddraft tubes and are tubes used in addition to the previously describeddraft tubes. The rejuvenation tubes are pipes 3″ in diameter and 31 to32 feet long.

FIG. 5A illustrates the good results obtained with continuous hydrogenrejuvenation using generally one rejuvenation tube, but never more thantwo tubes. The numbers in boxes on the figure are material balanceserial numbers. The upper plot, FIG. 5A shows, between day 0 and day 3,the typical rapid catalyst deactivation that occurred in the hydrocarbonsynthesis reactor. FIG. 5B shows the number of rejuvenator tubes usedand the hydrogen flow rate. At about day 3, hydrogen was fed to onerejuvenation tube, first in the amount of about 2.5 kscfh and then atabout 5.5 kscfh (superficial gas velocity 37 cm/sec and 76 cm/sec). Assoon as the hydrogen gas was started to the rejuvenator tubes, not onlydid the catalyst activity cease to decline but it immediately began toclimb sharply. Although the activity varied somewhat during the rest ofthe run, depending upon what other experiments were being carried out(see Example 2), the catalyst activity remained at or near its maximumvalue, thereby demonstrating the efficacy of continuous rejuvenation.

The extent of the rejuvenation reaction occurring in the continuousrejuvenation experiment was monitored by thermocouples placed inside oneof the rejuvenation tubes. The measured temperature profile in therejuvenation tube is compared with the temperature profile in thereactor slurry surrounding the rejuvenation tube for three differentbalances in FIGS. 6(A-C). In FIGS. 6(A-C), the solid circles representthe axial temperature profile in the reactor slurry, while the opensquares represent the axial temperature profile in the rejuvenationtube. In all three plots of FIGS. 6(A-C), the abscissae represents theaxial distance in feet above the bottom of the reactor, and the ordinaterepresents temperature in degrees Fahrenheit. The difference intemperature between the rejuvenation tube and the reactor slurry is thetemperature rise in the rejuvenation tube, which is a measure of theextent of the rejuvenation reaction occurring there. A comparison of thetop two plots (FIGS. 6A & B) shows the effect of the temperature levelupon the extent of reaction occurring in the rejuvenation tube for twobalances that were close to one another in time. Balance 37, made at anaverage temperature in the reactor of 428.5° F., showed a considerablygreater temperature rise in the rejuvenation tube than was exhibited inBalance 33 made at an average reactor temperature of 418.5° F. Thus, theefficacy of continuous rejuvenation depends upon the temperature levelin the rejuvenation tube, which in these experiments was controlled bythe temperature in the reactor slurry itself. Monitoring temperature inthe reactor and temperature rise in the rejuvenator tube is an efficientmethod for monitoring catalyst rejuvenation. This temperature riseincreased as the amount of rejuvenation that occurred in therejuvenation tube increased.

A comparison of the bottom two plots in FIGS. 6B & C demonstrate thatthe condition of the wax in the reactor also affects the extent ofreaction occurring in the rejuvenation tube. Balance 24, made at 3.2days on synthesis, occurred at the beginning of the continuousrejuvenation experiment. Hence, for this balance, catalyst activity waslow being 4.3 (see FIG. 5 at day 3.2) indicating that the concentrationof the deactivating species in the wax and on the catalyst was high, andthe amount of reaction that occurred in the rejuvenation tube was alsohigh, as attested by the temperature rise in the rejuvenation tube. ForBalance 37 made at 5.43 days on synthesis, on the other hand, for whichthe reactor temperature was very similar to that for Balance 24 (428.5°F. vs. 430.3° F.) but for which the catalyst activity was near itsmaximum being 7.6, indicating that the level of deactivants in the waxand on the catalyst was low, the temperature rise in the rejuvenationtube was lower than that for Balance 24. Thus, both wax condition andtemperature level in the rejuvenation tube were important in determiningthe amount of the cleansing, rejuvenation reaction that occurred incontinuous rejuvenation.

The gas rate to the rejuvenation tubes, which determines the residencetime of the reactor slurry in the tubes, was also found to be importantin determining the efficacy of the rejuvenation. Referring again toFIGS. 5A & B, between days 10 and 11, rejuvenation was carried out inboth one and two rejuvenation tubes. While activity was declining withonly one rejuvenation tube in service, activity again increased when thesecond tube was put in service, even though the total amount of hydrogenbeing fed to the two rejuvenation tubes was held constant. These datashowed the importance of controlling the residence time of the fluids inthe rejuvenation tube by controlling the amount of hydrogen rejuvenationgas being fed to the tube. Rejuvenation time will, of course, bedependent on the degree of catalyst deactivation as well as rejuvenatortube diameter, and is within the control of the practitioner. Too high arate of hydrogen flow reduced the residence time in the tube to thepoint that insufficient time was available for the pre-cleanup andcleanup reactions to occur.

Reference to Table I reveals that even when only 1 rejuvenation tube of3″ diameter with hydrogen gas passing through it at 74.8 cm/sec. was inoperation there was a noticeable degree of improvement in catalystcirculation, Balances 47 and 58. During the period that there was nolift gas fed to the four lift tubes described in Example 2, but onerejuvenation tube of 3″ diameter was being used which employed H₂ atabout 72 cm/sec. superficial space velocity the catalyst distribution inthe vessel, as reflected by catalyst density measured at differentheights in the vessel, improved. This demonstrates, therefore thatcatalyst rejuvenation and improved catalyst distribution can be achievedin a single operation using the same tubes as both catalyst distributionand rejuvenation tubes when H₂ is used as the lifting gas.

Example 4

The following example gives the physical configuration and thebeneficial effects obtained from a rejuvenation tube that has externalinsulation and internal heating or cooling. The advantage is that theeffectiveness of continuous catalyst rejuvenation can be controlledindependently of temperature in the reactor.

Prior to Fun 16 of the hydrocarbon synthesis pilot demonstration unit(HCS-PDU), the rejuvenation tube was altered by providing (1) anexternal jacket for heat insulation and (2) an internal tube to provideeither heating or cooling. Specifically, the rejuvenation tube was anominal 3-inch diameter pipe 46′ in length. Attached concentrically tothe outside of this rejuvenation pipe was a nominal 4-inch diameter pipealso 46′ in length whose purpose was to reduce the rate of heat transferbetween the rejuvenation pipe and the reactor slurry. The ends of the4-inch pipe were sealed against the 3-inch pipe so that the gap betweenthem served as a heat transfer barrier. Inside the 3-inch rejuvenationpipe and concentric with it was placed a nominal 1-inch pipe the bottomof which was sealed. This pipe extended the length of and extended outof the top of the 3″ pipe and ended in a “T” fitting. This 1-inch pipecould be heated or cooled with steam to control the temperature of thefluid inside the rejuvenation pipe independently of the temperature inthe reactor. Steam was supplied to the upper end of the 1-inch pipethrough the horizontal arm of the “T” fitting. Inside the 1-inch pipeand concentric with it was placed a length of ¾-inch tubing thatextended to length of the 1 inch pipe and extended out of the top of the“T” and exited into a steam trap. The purpose of this tubing was toprovide an outlet for the condensate accumulating in the 1-inch pipe inthe event the 1-inch pipe was used for heating or to provide an outletfor the exit steam in the event the 1-inch pipe was used for cooling.The lower end of the 1-inch pipe was outfitted with six fins to increasethe transfer of heat to the incoming liquid-catalyst slurry. Each finwas ¼″ thick, ½″ wide, and 8′ long. The lower end of the rejuvenationpipe was baffled to prevent, as much as possible, influx of reactantgases with the liquid-catalyst slurry into the lower end of therejuvenation tube.

The following data show the benefit for heating the rejuvenation tube bycomparing the rate of change in conversion in the reactor with timebrought about by adding steam to heat the rejuvenation tube contents.During the period of these tests, all other conditions of feed rate,feed composition, temperature, and slurry height were kept essentiallyconstant. The results are given in the table below.

Benefit of Adding Heat During Continuous Rejuvenation

Heat Addition No Yes HCS-PDU Run 16 Balance Periods 19-23 24-35Rejuvenation Gas Rate, KSCFH  5.1  4.8 Average Reactor Temperature, ° F.427.7 427.3 Average Temperature in Rejuvenation Tube, ° F. 429.9 439.6CO Conversion Range, % 32-26 26-36 Conversion Change/Day −12.4 +10.4

In this run of the hydrocarbon synthesis pilot demonstration unit, fivebalances (19-23) were made in which catalyst-wax slurry was pumpedthrough the rejuvenation tube using pure H₂ as the rejuvenation and liftgas with no steam being added to the 1-inch pipe. The averagetemperature in the rejuvenation tube was 429.9° F. compared with anaverage reactor temperature of 427.7° F. The CO conversion during thisperiod dropped from 32 to 26% over the period of about a half day,giving a rate of change in conversion of −12.4% conversion/day. Thedegree of catalyst rejuvenation occurring was inadequate to maintain aconstant catalyst activity.

Then, over the next 12 balance periods, while keeping the rejuvenationgas rate nearly constant, high pressure steam was added to the 1-inchpipe through the horizontal arm of the “T” fitting to help heat thecontents of the wax-catalyst slurry being contacted in the rejuvenationtube with the hydrogen. The average temperature in the rejuvenation tubeduring this period was 439.6° F. in the rejuvenation tube compared withan average temperature of 427.3° F. in the reactor. Over about a 24-hourperiod, the conversion increased from 26 to 36%, giving a rate of changein conversion of +10.4% conversion/day. Thus, by being able to increasethe temperature inside the rejuvenation tube just about 10°-12° F.independently of the temperature in the reactor itself, catalystrejuvenation was increased to the point that not only did the conversionstop falling but rather it was increasing at a rapid rate.

These data confirm the earlier data presented in Example 3 thatdemonstrated the improvement in catalyst rejuvenation brought about byperforming the rejuvenation in the rejuvenation tube at a temperaturehigher than that of the surrounding reactor. The data presented heresupport the invention that by insulating the rejuvenation tube andproviding internal heating, the rejuvenation rate can be controlledindependently of the reactor temperature.

What is claimed is:
 1. A method for rejuvenating reversibly deactivatedparticulate hydrocarbon synthesis catalyst in a slurry phase reactor,said method comprising the use of substantially vertical draft tubemeans, open at both ends, fully immersed in the slurry containing thecatalyst and injecting a hydrogen containing gas at or substantiallynear the bottom of said draft tube means thereby lifting catalyst inslurry from the bottom of the slurry phase reactor into and through theopen bottom end of the draft tube means, rejuvenating catalyst in thepresence of said hydrogen containing gas in the vertical draft tubemeans and ejecting the rejuvenated catalyst into the top of the slurryphase in the slurry phase reactor through the open top of the draft tubemeans.
 2. The method of claim 1 wherein the cross sectional surface areaof the draft tube means occupies from 0.2 to 10% of the cross sectionalsurface area of the slurry phase reactor.
 3. The method of claim 1wherein the substantially vertical draft tube means comprises one ormore individual draft tubes.
 4. The method of claim 3 wherein whenmultiple draft tube means are employed no single draft tube constitutesmore than 50% of the total cross sectional area of the total draft tubemeans.
 5. The method of claim 3 wherein the individual draft tube meansof the draft tube means possess a diameter such that a sufficientlylower slurry density is maintained in the draft tubes than in thesurrounding slurry in the reactor vessel.
 6. The method of claim 1wherein the open bottom end of the draft tube means is located at aheight of from 0.1 to 1 foot above the bottom of the slurry phasereactor.
 7. The method of claim 1 wherein the hydrogen containing gas isinjected into the substantially vertical draft tube means at a rate suchthat the superficial gas velocity in the draft tube means is at least0.2 to 40 times the superficial gas velocity of the gases in the slurryphase reactor thereby permitting sufficient residence time in the drafttube means for catalyst regeneration - rejuvenation to occur.
 8. Themethod of claim 1 wherein the bottom end of the substantially verticaldraft tube means is fitted with gas deflecting means to minimize entryof reactant gases into the draft tube means.
 9. A method for producinghydrocarbons in a slurry phase reactor utilizing a hydrocarbon synthesiscatalyst which comprises rejuvenating said hydrocarbon synthesiscatalyst in accordance with the method of claim
 1. 10. The method ofclaim 9 wherein said slurry comprises the product of said hydrocarbonsynthesis including C₅₊ hydrocarbons.
 11. The method of claim 10 whereinsaid slurry comprises C₅ -C ₁₀₀ hydrocarbons.
 12. The method of claim 11wherein said slurry comprises Fischer-Tropsch waxes.
 13. The method ofclaim 9 wherein said hydrocarbon synthesis catalyst comprises aFischer-Tropsch catalyst.
 14. The method of claim 13 wherein saidFischer-Tropsch catalyst comprises a Group VIII metal selected from thegroup consisting of iron and cobalt.